Conventional Two-Fibre Transmission
FIG. 1 depicts a conventional two-fibre transmission link where blocks 101 and 102 can represent regeneration or central office sites. Connecting the two sites together is a fibre optic cable. Within the cable there are multiple strands of fibre 103, of which two have been shown. In this type of transmission system, communication from a transmitter (TX) at site A to a receiver (RX) at site B utilizes one signal wavelength (.lambda.1) and one strand of an optical cable. Communication in the opposite direction uses a different strand of the optical cable and the same, or different, wavelength (.lambda.2) to carry the signal.
Referring again to FIG. 1, sites A and B (101 and 102) can represent different site configurations. In one configuration, one terminal site might communicate directly to another terminal site in a complete end-to-end, communication system. Alternatively, FIG. 1 could represent a single link in a longer chain of transmission stations. In other words, sites A and B might be representative of a site C and a site D and a site E and so on, until a final site containing terminating transmission equipment is reached.
Depending upon the wavelength chosen for transmission, the strand of optical fibre 103 used may exhibit different attenuation characteristics which may limit the possible sparing of regenerator sites, e.g., sites A and B. Attenuation in a typical single-mode optical fibre is about 0.35 dB/kilometer at 1310 nanometer (nm) and about 0.25 dB/kilometer at 1550 nm. Thus, for systems operating at data rates of a few gigabits per second, regenerator sites could be spaced anywhere from about 35 to 45 kilometers when operating at 1310 nm and into the 70 to 80 kilometer range when operating at 1510 nm.
Wavelength-Division Multiplexer (WDM) Filters FIG. 2 depict a conventional narrow-band wavelength-division multiplexing communication system. Here, the term "narrow-band" is used to mean that more than one wavelength is utilized within the same transmission "window" of the optical fibre. For example, if the system is operating within a 1550 nm window, two signaling wavelengths of 1533 and 1557 nm might be used. For standard single mode fibre, the two main transmission "windows" of interest are 1310 nm and 1550 nm. Unlike the configuration shown in FIG. 1, communication between site A and site B in FIG. 2 is provided by a single strand of optical fibre 103. Bi-directional transmission is achieved through the utilization of wavelength-division multiplexing (WDM) filters, 201 and 203. (The devices 201 and 203 can be the same or slightly different devices, depending upon the manufacturing technique used to create them.) The purpose of WDM filters is to couple multiple wavelengths into (hereafter referred to as `on`) and out of (hereafter referred to as `off`) the transmission fibre. In the example shown, WDM filters 201 and 203 couple the two wavelengths 1557 and 1533 nm on and off a single fibre 103 of a fibre optic cable.
WDM Technology
There are several technologies that can be used to construct WDM filters. For example, etalon technology, defraction grading technology, fused biconic taper technology, and holographic filter technology. One technology that has proven to be widely useful in the telecommunications industry is dichroic filter technology. This technology offers wide channel passbands, flat channel passbands, low insertion loss, moderate isolation, low cost, high reliability and field ruggedness, high thermal stability, and moderate filter roll-off characteristics.
An illustrative example of a conventional three-port dichroic filter 300 is shown in FIG. 3. A dichroic filter is comprised of one or more layers of dielectric material coated onto a, for example, glass substrate 305 with lenses 310 to focus the incoming and outgoing optical signals. The choice of dielectric material, the number of dielectric layers coated onto the substrate, and the spacing of these layers are chosen to provide the appropriate transmissive and reflective properties for a given--target--wavelength. For example, if .lambda.1 is the target wavelength to be transmitted through the filter, the number and spacing of the dielectric layers on the substrate 305 would be chosen to provide (1) a specified passband tolerance around .lambda.1 and (2) the necessary isolation requirements for all other transmitted wavelengths, for example, a wavelength, .lambda.2, transmitted by a second transmitter.
The dichroic, or WDM, filter is constructed by placing self-focusing lenses, such as "SELFOC" lenses 310, on either side of the dielectric substrate 305. "SELFOC" lens 310 focuses incoming light (.lambda.1 and .lambda.2) to a particular location on the dielectric substrate.
Attached to the "SELFOC" lenses through an adhesive bonding process are, typically, single-mode optical fibers. For convenience, the locations at which optical fibers attach to the "SELFOC" lenses 310 are called ports: port 1320, port 2325, and port 3330. Connected to the ports are optical fibers 335, 340, and 345 respectively.
For example, all of the fight (comprised of .lambda.1 and .lambda.2) passing through fiber 335 connected to port 1320 is focused by lens 310 to a single location on the dielectric substrate 305.
Since the substrate is coated to pass wavelengths around .lambda.1, virtually all of the light at .lambda.1 passes through the dielectric substrate 305 and, via the second "SELFOC" lens, is collimated into port 3330, and passes away from the filter on optical fiber 345. Any other wavelength incident on the filter through port 1320 (e.g., light of wavelength .lambda.2) is reflected off the multilayer substrate, focused back through the first "SELFOC" lens to port 2325, and passes away from the filter on optical fiber 340. Likewise, the filter performs the same function for light traveling in the opposite direction.
This technology could be used to, for instance, implement WDM filter 201 shown in FIG. 2.
FIG. 4 is a variation of the system shown in FIG. 1, a two-fiber design where one wavelength (.lambda.1) is transmitted on one fiber in one direction, and another (or possibly the same) wavelength (.lambda.2) is transmitted on the other fiber in the opposite direction. Erbium-doped fiber amplifiers (EDFAs) can be deployed along such a link in multiple locations: immediately following the transmitter (TX), making them post-amplifiers; immediately preceding a receiver (RX), making them pre-amplifiers; or between a transmitter and receiver, as shown in FIG. 4, making them line-amplifiers. Commercially available EDFA devices only operate in the 1550 nm window. Typically, in the line-amplifier configuration, regenerator spacing can be almost doubled, from approximately 70 to 80 kilometers to approximately 140 to 160 kilometers. (This analysis assumes typical filter attenuation and that at 80 kilometers the system is attenuation limited and not dispersion limited for distances less than 160 kilometers). Hence, if the cost of two EDFAs is less than the cost of a conventional fiber optics transmission system regenerator, the two EDFAs 401 and 403 can be used to reduce equipment deployment costs when constructing a transmission network such as that shown in FIG. 4.
Illustrative Systems
FIG. 5 depicts one configuration for a dual wavelength, bi-directional narrow-band WDM optical amplifier module, 901. Components used to construct the amplifier module 901 include: two WDMs, 201 and 203 (input and output ports of the amplifier module), and two EDFAs, 903 and 905, which can be either single-pumped or dual-pumped depending upon the communication system's power constraints/requirements. This line-amplifier configuration extends the regenerator spacing while providing bi-directional transmission utilizing a single-fibre strand of the cable facility 103.
It should be noted that the amplifier module 901 can be cascaded to extend even farther the distance between site A and site B. (The number of amplifiers that can be cascaded, between sites A and B, is limited by the dispersion characteristics of the transmission equipment deployed at sites A and B.)
Referring now to prior art FIG. 6, U.S. Pat. No. 5,452,124 describes a bi-directional amplifier module design that can be constructed utilizing a single EDFA. In this configuration, bi-directional transmission over a single optical fibre is achieved using four WDM filters. All signal wavelengths must pass unidirectionally through the EDFA 401 due to the constraint of using optical isolators in the EDFA 401 (refer to FIG. 5). Therefore, the two transmission wavelengths traveling in opposite directions, must be broken apart and recombined through WDM filters to pass unidirectionally through the EDFA. Similarly, the two amplified wavelengths must be broken apart and recombined through WDM filters to continue propagating toward their respective receiver sites. WDM filter 203 is constructed to bandpass 1557 nm and WDM filter 201 is constructed to bandpass 1553 nm.
Assuming a typical 1550 nm EDFA operational band, then going through FIG. 6 in a left-to-right direction we see a 1557 nm signal is transmitted from site A 101, through the east WDM filter 203, and onto the fibre cable 103. As the signal enters the amplifier module it is separated by the west WDM filter 201. (Each WDM filter in FIG. 6 has its external connection points labeled either 33 or 57. Connections labeled 33 carry optical signals at the 1533 nm wavelength. Connections labeled 57 carry optical signals at the 1557 nm wavelength.) The signal then travels to the east WDM filter 203 where it is routed into the EDFA amplifier 401. Upon leaving the EDFA, the 1557 nm signal is routed by another west WDM filter 201 to the amplifier module's output east WDM filter 203 where it is placed onto the fibre optic transmission cable 103. Finally, the signal leaves the transmission cable 103, enters the west WDM filter 201 at site B 102, and is routed to that site's receiver equipment. Signals transmitted from site B, at 1533 nm, take a different path through the WDM filters 201 and 203 and EDFA 401 on their way to site A's receiver. An advantage of this prior art embodiment over the configuration described in the earlier prior art of FIG. 5 is that only a single erbium-doped fibre amplifier is required. Because multiple wavelengths are being amplified by a single amplifier, it is sometimes preferable that the EDFA 401 in FIG. 6 uses a dual-pumped amplifier rather than a single-pumped amplifier. The additional gain provided by a dual-pumped EDFA could compensate for the signal strength lost by virtue of passing it through a number of additional elements.
For some time now, in North America, dense wavelength division multiplexed (WDM) systems having a plurality of channels transmitted on a single optical fibre have been used primarily in long-haul, backbone, Trans-Canada, Trans-United States systems. For example, between major cities in the United States and between major cities in Canada, there are fibre optic backbone routes several hundred kilometers long, having optical fibre amplifiers disposed periodically along these routes, wherein different channels are transmitted at different wavelengths on a single optical fibre.
In larger cities, for example in Toronto, large central offices exist having fibre optic links therebetween, and in some instances complicated mesh structures of optical fibre links exist between some of these central offices. It is also common for fibre optic cables to be provided from these central offices that offer high bit-rate links routed directly into office buildings via an optical fibre carrying data to and from their local PBX. Hence, fibre optic links exist from central office to central office and from central office trunks to private networks.
Currently, many such local installations do not support multi-wavelength multiplexed signals. These local installations are typically in the form of 1310 nm signals in one direction and 1310 nm signals in the other direction, similar to what is shown in FIG. 1, but wherein both optical fibres transmit and receive the same wavelength.
As of late, there is growing concern relating to utilization of optical fibre cable. The installation of additional optical fibre cables is a costly proposition. For example, on long-haul routes, right of ways must often be established and special trains capable of plowing beside a railway route are often required to add new cable on existing routes.
Thus, on long haul routes, between cities, wavelength division multiplexing has become an economically viable alternative.
However, within metropolitan areas, typical central offices may be 20 kilometers apart or less, and regenerators are not required. Adding new cable is a variable cost by length, and adding a short length has generally been considered more economically viable than adding wavelength division multiplexors and demultiplexors which are considered to be a fixed cost per channel.
So currently and in the past, it has been less expensive to provide short lengths of cable when required, than to implement a WDM system.
Notwithstanding these factors, there is now some interest in using multichannel technology. For example, when a new customer would like a connection to a central fibre cable, and the number of central fibres is limited, WDM systems are being considered. In this instance where some part of the trunk (central fibre cable) cannot support the branches (the customers) demanding the service, there exists a need for a cost effective WDM system.
It is an object of this invention to provide such a system.
It is an object of this invention to provide an optical transmission system wherein uncooled lasers are provided designed to operate in the absence of optical circulators such that the system is generally tolerant of back reflections that would otherwise cause a system having narrower channels to suffer from laser mode hop.
It is a further object of this invention to provide an evolvable system which as it evolves becomes gradually more expensive but at the same time is cost effective as the number of subscribers increases beyond some predetermined number.
Hence it is an object of this invention to provide a WDM system that utilizes inexpensive commercially available uncooled lasers.